The present invention relates to a filtration apparatus for separating suspended solids from a liquid suspension and more particularly, to a method and a filter apparatus having an enclosure around the filter screen with an elevated outlet to assure the even depositing of suspended solids from the liquid being filtered onto the filter screen.
This invention is related to the filter apparatus set forth in U.S. Pat. No. 4,622,144 assigned to the assignee of the present invention.
Such filters typically comprise a vertical cylindrical screen pressure vessel body lined with filter media. The filter media is sealed to the filter body above and below the screen section. When liquid to be filtered is pumped into the filter, the liquid flows through the filter media and screen section, leaving the dirt behind on the inner surface of the media. If the dirt load is free flowing and therefore causes little resistance to flow, a large amount of cake can accumulate in the lower portion of the filter. As the cake increases in thickness in the lower portion of the filter, resistance to flow increases so that the liquid level in the filter rises until it reaches the top of the filter and then a cake starts to accumulate at the top of the filter. The result is, for a freely-flowing dirt load, a cake that is very heavy at the bottom of the filter, tapering up to a very light cake at the top of the filter.
This shape of cake is undesirable for several reasons. One is that it interferes with the dewatering action of the membrane. Since the cake that the membrane is squeezing is tapered, the squeezing action is not uniform and the cake is likely to contain more moisture at the bottom than at the top.
In addition, a tapered cake is more difficult to break and discharge from the filter. A completely free-flowing cake may all accumulate at the bottom of the filter and completely disrupt the ability of the filter to discharge the cake.
In accordance with the present invention, a new and novel liquid suspension filtration method and apparatus are provided for removing suspended solids from liquid and depositing the solids on a filter media or wire screen through which the liquid suspension carrying the solids has passed.
This is accomplished generally by introducing a liquid suspension into a pressure vessel having a filter wall section on which the solids are deposited. The vessel is filled so that the entire surface area of the filter section is covered. As the liquid flows through the filter wall section, the suspended solid is deposited and accumulated thereon with the filtrate passing therethrough.
To assure the even depositing of suspended solids on the filter wall section, the vessel includes an enclosure wall outside the filter section forming an annular outlet chamber. The outlet chamber is in enclosed contact with the lower portion of the vessel, and includes an outlet near its upper portion. Accordingly, liquid passed through the filter section must rise in the outlet chamber to the outlet level before exiting the filter vessel. Accordingly, the solids accumulate on the filter section in an even manner and thickness. When such solids are of such a thickness that further filtration is no longer feasible, flow of liquid into the vessel is discontinued and pneumatic pressure means are actuated to create a pneumatic pressure within the vessel. This causes further flow of liquid through the accumulated solids on the filter section and gradually displaces the liquid suspension from the filter section to expose increasing areas of accumulated solid to the pneumatic pressure. The pneumatic pressure acting on the exposed solids is operative to compress or squeeze the solids against the filter section and remove additional liquid therefrom to form a layer in the nature of a cake. When the excess liquid is displaced from the filter section, the remaining liquid suspension is removed from the vessel and the pneumatic pressure is released. Thereafter the cake is removed from the septum and discharged from the vessel.
The above described arrangement is suitable for the extraction of liquids from suspended solids which are substantially air impervious when separated and deposited on the filter section. However, should the separated suspended solid not have the impervious characteristic, the arrangement of the present invention may be provided with a flexible fluid impervious curtain which overlies the filter section. The curtain is constructed and arranged so as to be spaced from the filter section to permit the passage of liquid through the latter. Upon the application of the pneumatic pressure to gradually displace the liquid from the filter section, the flexible curtain is forced into face to face engagement with the solids accumulated on the filter section by the pneumatic pressure acting thereon to compress the solids and extract liquid therefrom in substantially the same manner as described above.